The Difference Between Migration Flow and Migration Stock, in Terms

The difference between migration flow and migration stock, in terms of measuring and observing immigration as a social phenomenon, is

A) flow describes immigrants with specialized and unique skills that the native workforce does not have, while stock describes those who have few specialized skills
B) flow describes those who are first-generation immigrants, while stock describes those who intend on remaining in the nation
C) flow describes immigrants who are in a nation for a specific purpose for a short term, such as vacation, a business trip, or studying abroad; stock describes those who are in the nation long-term for work.
D) flow describes the number of people coming and leaving another country given period of time, while stock describes the total number of immigrants in a nation in a particular moment